How can a custom icon be loaded in an add-on?
You can only use already available icons, too see which are available enable the Icons add-on. A panel with all icons will be displayed in the text editor properties (or python console). The name of the icons is shown in a tooltip.

You can use the icon attribute like:

row = layout.column(align=True)
row.operator('id_name', icon='QUESTION')


 

Custom icons require compilation which is off-topic here but you could find a discussion on custom icons at this BA thread.

Possibly outdated information on how to integrate custom icons you find in the Wiki. There is descibed that all icons are stored in a single .png file which explains that you can't use SVG or any other format.